1. Field of the invention
The invention relates to a method and an apparatus for filtering the luminance signal out of a CCVS television signal.
2. Description of the Prior Art
In the color signal part of a color television receiver, the video frequency CCVS (Composite Color Video Signal) is split into the luminance signal (light density signal) and the chrominance signal (color hue signal). The chrominance signal is then demodulated according to the color demodulation method (PAL, SECAM, NTSC) and these color signals and the luminance signal are fed to the picture tube through a matrix.
The separation of the chrominance components of the CCVS-signal required for obtaining the luminance signal is customarily accomplished in a "color trap" which sufficiently suppresses the color spectrum from 3.1 to 5 MHz. An LC-turned circuit is usually used as the color trap. This may be a series-resonant circuit connected into the collector circuit of a transistor driven by the CCVS-signal, the band width of which can be adjusted by a shunt resistance.
Such filters constructed from coils and/or capacitors, however, have a non-linear phase response, which leads to a distortion of the pulse response which is visible in the picture. Firstly, these filters have a poor time behavior, since each luminance jump excites the tuned circuit to perform a damped oscillation. Secondly, troublesome transients also occur if the phase of the color carrier is changed heavily, such as in the transition from green to purple.
If expensively constructed quadrupoles are used as the color trap, the time behavior of the luminance channel is only slightly improved.